This invention relates to semiconductor lasers, and in particular, though not exclusively, to a semiconductor laser which uses a combination of gain profiling, and Quantum Well Intermixing (QWI) and advantageously a Wide Optical Waveguide (WOW) to provide a high power semiconductor laser device which has relatively high brightness and good beam quality.
Semiconductor lasers are commonly used in a number of applications, e.g. computer CD ROMs and compact disc players. High power semiconductor lasers are also used in solid-state laser pumping materials processing and medical applications. A semiconductor laser producing more than a few hundred milliwatts of light is normally termed a xe2x80x9chigh powerexe2x80x9d device.
Previous high power semiconductor laser devices have suffered from a number of problems such as poor beam quality and low brightness. The output power is also limited due, for example, to interactions between the optical field and the laser facet (mirror).
The laser facet is cleaved semiconductor and as such contains a high density of vacancies and broken bonds which can lead to the absorption of generated light. Light absorbed at the laser facet generates heat as excited carriers recombine non-radiatively. This heat reduces the semiconductor band-gap leading to an increase in absorption inducing thermal runaway which may result in Catastrophic Optical Mirror Damage (COMD).
Many schemes have been suggested and implemented to increase COMD levels. These, for example, include facet passivation by chemical treatments and band-gap widening in the mirror regions. Band-gap widening can be achieved by re-growth processes. However, all of these schemes have proved complicated and unreliable with no single process being widely adopted.
Therefore, to produce high powers without suffering from COMD, manufacturers have previously tended to increase the width of the laser aperture. Although this increases the overall power output of the semiconductor laser, the amount of power per unit width emitted from the laser aperture is in effect reduced. Further, although this method does produce higher power, it is accompanied by a number of other disadvantages. These disadvantages include a reduction in the brightness of the device, a reduction in the quality of the laser output beam (i.e. loss of spatial coherence), and it is also more difficult to dissipate heat out of the active region of the device
One of the reasons why the beam quality of previous high power devices is poor is due to the interaction of carriers with light in the active region of the device. These interactions take the form of spatial hole burning and self-phase modulation, which tend to induce changes in the refractive index. These changes in the refractive index allow modes higher than the fundamental mode to propagate resulting in a break-up of the near-field (filamentation) and hence broadening of the far-field.
It is an object of at least one aspect of the present invention to obviate or at least mitigate one or more of the aforementioned problems and/or disadvantages of the prior art
It is a further object of at least one aspect of the present invention to provide a semiconductor laser device which has a relatively high brightness and good beam quality as compared to previously known semiconductor laser devices.
SUMMARY OF INVENTION
According to a first aspect of the present invention there is provided a semiconductor laser device including at least one portion which has been Quantum Well Intermixed (QWI), and means for providing gain profiling. Herein the term gain profiling is meant to mean alteration of a profile of a concentration of carriers within an active portion or region of the device. This is in contra distinction to prior laser devices where current injection is substantially uniform or constant across the active region.
This combination of techniques produces a high power device with low loss integrated spatial filters.
The device of the present invention therefore provides relatively high power and high brightness vis-a-vis prior devices.
Advantageously the device also provides a wide optical waveguide (WOW). Herein the term WOW is meant to mean a waveguide which supports more than the fundamental mode.
Preferably, the laser device is fabricated at least partly from a compound semiconductor material.
Preferably, the semiconductor device is fabricated from a III-V semiconductor based materials system, eg a Gallium Arsenide (GaAs) or Indium Phosphide (InP) system.
Preferably, the semiconductor device is fabricated at least partly from Aluminium Gallium Indium Phosphide (AlGaInP).
Preferably, the semiconductor laser device comprises a multiple layer wafer structure.
Preferably, the multiple layer wafer structure includes an optical waveguide preferably comprising an undoped high refractive index core region containing at least one Quantum Well (QW) as-grown, and bounding the core region doped cladding regions having lower refractive indices then the core region, and advantageously a further p++ contact layer.
It is further preferred that the laser wafer structure is grown on a (100) Si doped GaAs substrate misorientated 10xc2x0 to the [111] A direction.
Preferably, the at least one Quantum Well (QW) layer comprises at least one Quantum Well (QW) layer, and in one embodiment comprise a double Quantum Well (QW) layer of around 670 nm emission wavelength.
The laser wafer structure may be grown by any suitable III-V semiconductor growth method. It is preferred that the laser wafer structure is grown by Metal-Organic Vapour Phase Epitaxy using a large III-V growth ratio or Molecular Beam Epitaxy (MBE).
Preferably the multiple layer wafer structure consists of an n-doped GaAs buffer layer, an n-doped low refractive index waveguide cladding layer, an undoped high refractive index waveguide core layer, a p-doped low refractive index cladding layer, a p-doped low index barrier reduction layer, a p++ doped GaAs capping layer, a dielectric insulation layer and a p-type contact.
In one embodiment the multiple layer wafer structure consists of a 500 nm Silicon (Si) doped (3xc3x971018 cmxe2x88x923) GaAs buffer layer, a 1.0 xcexcm Si (6xc3x971017 cmxe2x88x923) doped (Al0.7Ga0.3)0.5In0.5P lower waveguide cladding layer, a 600 nm undoped (Al0.3Ga0.7)0.5In0.5P waveguide core layer, a 1.0 xcexcm Zinc (Zn) (6xc3x971017 cmxe2x88x923) doped (Al0.7Ga0.3)0.5In0.5P cladding layer, a Zn (2xc3x971018 cmxe2x88x923) doped Ga0.5In0.5P barrier reduction layer and a 300 nm Zn ( greater than 1xc3x971019 cmxe2x88x923) doped GaAs capping layer.
Preferably, there are a number of low band-gap Quantum Wells (QWs) substantially centrally provided in the undoped waveguide core layer as-grown.
Preferably, the low band-gap Quantum Wells (QWs) comprise two strained 6.8 nm wide Ga0.5In0.5P Quantum Wells (QWs) and an undoped layer therebetween comprises a 15 nm (Al0.3Ga0.7)0.5In0.5P barrier layer.
In a preferred form the device consists of at least three distinct portions:
first and second at least one portions which are Quantum Well Intermixed (QWI) and optically passive, and
a mid portion between the first and second at least one portion which is optically active and includes at least one Quantum Well (QW).
The mid portion therefore has a band-gap equivalent to the multiple layer wafer structure, as-grown, while the band-gap of the first and second at least one portions are blue shifted, which makes the first and second at least one portions substantially transparent to light (optical radiation) generated in the active mid portion.
Preferably, there is provided means for injecting current into the mid portion thereby providing the optical gain profiling in the device, in use.
It is preferred that the current injection means is a contact shaped as a geometric pattern, wherein the shape of the contact is selected to allow for matching of an optical mode and gain of the structure.
The contact may be shaped substantially as a half-tone pattern, finger pattern, triangular pattern or Gaussian distribution pattern.
In one form the first at least one portion is relatively short, for example, 1 to 100 xcexcm, and acts in use as a Non-Absorbing Mirror (NAM) allowing high output powers at the device facet, and the second at least one portion is relatively long, for example, around 1 xcexcm, and acts in use, as a spatial filter
In another form both first and second at last one portions are relatively short whereby both first and second at least one portions act as NAMs and even higher power outputs are obtained. Preferably, the relatively short first and second at least one portions are 1 to 100 xcexcm long.
According to a second aspect of the present invention there is provided a semiconductor laser device providing gain profiling and Quantum Well Intermixing (QWI).
According to a third aspect of the present invention there is provided a method of fabricating a device according to either the first or second aspect of the present invention, the method comprising the steps of:
(a) providing a, laser device body portion including at least one Quantum Well (QW);
(b) defining on the device body portion at least one portion to be intermixed and intermixing the Quantum Well(s) (QWs) within the at least one portion, and further
(c) defining on the device body portion at least one optically active region and providing current injection means associated with the optically active region.
Step (b) may be undertaken before or after step (c), though preferably before.
The intermixing step may be selected from a number of QWI techniques, eg Impurity Induced Disordering or preferably Impurity Free Vacancy Disordering (IFVD). In the latter case, preferably the process includes deposition of a dielectric layer, eg a Silica (SiO2) layer, subsequent rapid thermal annealing causing semiconductor material to dissolve into the Silica thereby leaving vacancies in the semiconductor material.
According to a fourth aspect of the present invention there is provided an apparatus including at least one device according to either of the first or second aspects of the present invention.
The apparatus may comprise a CD ROM or CD player or a telecommunications apparatus.
According to a fifth aspect of the present invention there is provided a system including at least one device according to either of the first or second aspects of the present invention.
The system may comprise a telecommuncations system.